const point = [378395.25, 5824766] // easting: 378395.25 northing: 5824766

const epsg25833 = require('epsg-index/s/25833.json');
const epsg4326 = require('epsg-index/s/4326.json');
proj4.defs([
  [
    'EPSG:25833',
    epsg25833.proj4
  ],
  [
    'EPSG:4326',
    epsg4326.proj4
  ]
]);
const source = new proj4.defs("EPSG:25833");
const dest = new proj4.defs("EPSG:4326");
proj4(source, dest, point);